1. Field of the Invention
This invention relates to a prosthetic patello femoral joint assembly.
2. Description of the Prior Art
The patello femoral joint experiences very large contact forces in balancing the flexion moment across the knee. In ascending and descending stairs, the contact force on the patello femoral joint has been biomechanically calculated as lying in the region up to six times body weight.
Previous designs of patello femoral articulation have compromised between the contact conformity of the patella articulating surface against the femur and the need to accommodate variations introduced by surgical inaccuracy, patient anatomical variation and the movement required from the articulation without inducing averse fixation forces or inappropriate contact conditions.
In general, due to the lack of precise datums, it is very difficult surgically to arrange for the rotational positioning of the patella component so that it precisely matches the orientation of the patello femoral groove geometry on the femoral implant component. The majority of designs in the market place therefore use an axisymmetric configuration for the bearing surface of the patella component. This inevitably results in low conformity between this part and the femur. The maximum conformity possible being a line contact. This results in high stresses in the plastic articulating material of the patella component with premature wear. Alternative designs with saddle shaped articulations on the patella component have been prone to a different type of problem. These components theoretically allow for a larger contact surface between the patella component and the femur, but are extremely sensitive to alignment. When surgical accuracy is not absolute, the error in alignment causes edge loading, and excessive fixation loading of the patella component.
Designs of patella resurfacing components which incorporate a trunnion to allow the appropriate rotational orientation of a more conforming patella bearing surface have been proposed. These include the designs of Buechal and Papas (U.S. Pat. No. 4,309,778). Nevertheless, these devices still have a high dependence on surgical accuracy to ensure that the medio lateral positioning of the patella component, and the relative inclination of the component to the transverse plane of the patella bone, is appropriate. The currently proposed invention aims to overcome these problems.